Chromium plating



400 ,y goo 6'00 INVENTOR.

FPa/Zz'e I"? Qaflwrzz ATTORNEY R. P. DURHAM CHROMIUM PLATING Original Filed Dec. 18, 1959 V M A June 8, 1965 p w m United States Patent 5 Claims. c1. 29-1835) This application is a division of my copending United States patent application Serial No. 860,436, Rollie P. Durham, entitled Chromium Plating, and which is a continuation-in-part of an earlier United States patent application Serial No. 812,081.

This invention relates to electroplating and more particularly to chromium plated articles of exceptional durability and to a method of forming such articles.

Since the initial use of chromium in decorative plating, the problem of obtaining decorative coatings of satisfactory durability has been a problem. Moreover, for unknown reasons the quality of a chromium coating will vary from time to time even though the coatings are deposited from the same solution apparently under exactly the same conditions.

Accordingly, it is a primary purpose of this invention to provide decorative chromium plated parts which exhibit vastly improved durability and to provide a method of forming such articles with an unusual degree of consistency.

In general my invention involves forming decorative chromium plated parts by applying at least two chromium layers of different characteristics to a suitably prepared surface of a part. A careful selection of the individual layers used in accordance with the invention permits the consistent formation of chromium plated parts which are not only more durable than conventional chromium coatings but also permit chromium plated steel parts to be produced which do not fail to red rust under comparatively severe corrosion testing.

The preparation of the part prior to applying my chrominum coating is as important to my invention as it is to conventional chromium plating. Accordingly best durability has been obtained when protective surface coatings interjacent the chromium coating and the surface of the metal part are used. For example, I prefer to apply the dual layer film to a steel part which has been coated successively with copper and nickel in the known and accepted manner of preparing steel parts for chromium platmg.

As a specific example of the practice of my invention a metal part, such as one made from SAE 1010 steel, is polished and cleaned to form a. suitable surface for subsequently applied electrodeposited coatings. The particular manner in which the metal part is cleaned is only as material to my invention as it is to any general electroplating procedure and may, therefore, be varied considerably.

In one method of cleaning, the workpiece is anodically treated in an alkaline solution which is made up as follows:

Oz./ gal. Sodium orthosilicate 8 Tetrasodium pyrophosphate 1 Sodium carbonate 1 Patented June 8, 1965 "ice 40 seconds in a room temperature aqueous bath containing 25% muriatic acid, by volume.

Subsequently the part is removed from the muriatic acid bath and thoroughly rinsed. The part is then anodically cleaned a second time in the manner described above but only for approximately 30 seconds. It is then removed from the anodic cleaning bath and again rinsed and immersed for about 10 seconds in the muriatic acid bath and then thoroughly rinsed.

After cleaning, the workpiece is preferably immersed in a copper or nickel strike solution to insure good adherence of subsequently electro-deposited coatings. One such solution which can be employed to apply a thin coating of copper to the part contains the following ingredients:

Oz./ gal. Copper cyanide 2-4 Sodium cyanide (free) 1.5-2 Caustic soda 2-3 A negative potential is applied to the part for a sufiicient duration approximately 170 seconds, under a cathode cur rent density of approximately 20 amperes per square foot to 25 amperes per square foot in order to apply about .0001 inch of copper.

The part is subsequently removed from the copper strike bath, rinsed and then dipped in a dilute aqueous solution of sulfuric acid, 5% H by volume. Immersion for about 10 seconds in this bath, which is at room temperature, aids in keeping the copper surface active to improve adhesion of the subsequently applied thicker copper coating. The part is then removed from the sul furic acid bath, rinsed and immersed in an acid copper plating solution. A suitable acid copper bath has a composition as follows:

With air and cathode rod agitation, direct electric current is passed through the solution and copper anodes so as to impose an negative potential on the workpiece for a sufficient duration to deposit a copper coating of at least about 0.0015 inch. The bath is preferably maintained at a temperature of approximately 70 F. to F. while the cathode current density is maintained at approximately 50 amperes per square foot.

Although in some instances the workpiece can be nickel plated directly after removal, most generally it is preferred to bufi the copper coating bath prior to immersion in a nickel plating solution. After bufiing, of course, the workpiece must be cleaned again to prepare the copper surface for reception of the nickel plate.

As the initial cleaning, this latter cleaning can be accomplished satisfactorily in the normal and accepted manner for such cleaning, such as by a cathodic electrolysis, an anodic electrolysis, and an acid dip. Thus, after buffing the part is electrolyzed for 30 seconds to 60 seconds in an aqueous bath having about 2 ounces per gallon to 6 ounces per gallon of a mixture containing, by weight, about 55% soda ash, 25 trisodium orthophosphate and 20% caustic soda. Using a bath temperature of about F., a negative potential is imposed on the workpiece sufiicient to cause a cathode current density of about 50 amperes per square foot.

The part is then anodically cleaned in the manner described above for about 30 seonds, rinsed and then dipped in a dilute, sulfuric acid solution (1% H 80 by volume, in water) for 20 seconds to 30 seconds.

After immersion in the dilute acid solution, the part was thoroughly rinsed and immersed in a bright nickel electroplating tank. One such solution which is suitable 3 for depositing bright metal has the following ingredients:

Nickel sulfate (hydrated) oz./gal 3050 Nickel Chloride (hydrated) oz./gal 4-10 Boric acid oz./gal 5-8 Wetting agent oz./gal 0.00566 Primary brightener g./l .2-5 Secondary brightener mg./l 2-100 Direct electric current is passed though the solution with cathode rod agitation so as to impose a negative potential on the part for a sufficient duration to deposit a nickel coating of at least about 0.0005 inch. The bath is preferably maintained at a temperature of approximately 120 F. to 150 F. and the cathode current density is maintained at approximately 40 amperes per square foot to 60 amperes per square foot.

When a sufiicient thickness of nickel has been deposited on the part, it is removed from the nickel electroplating bath, rinsed and placed in contact with a suitable chromium plating solution.

With the preliminary copper and nickel layers applied, a plurality of chromium layers is then deposited thereover. The use of a plurality of chromium layers to improve durability of a decorative chromium plated part is known in the art. However, the plurality of layers applied by prior procedures does not present the material improvements in durability such as are obtainable with my invention. I have found that remarkable increases in durability of the part are obtained when the part is coated with a plurality of chromium coatings having different characteristics.

It is well known that chromium layers deposited from different bath compositions and even from the same bath under difierent conditions may have distinctly different characteristics. I have found that some combinations of chromium layers having certain characteristics will produce a remarkably unexpected improvement in corrosion resistance not obtainable by other combinations of chromium layers. For example, I have found that the marked increases in corrosion resistance can be obtained in a dual layer chromium coating if the first layer (Layer A) is electrodeposited from a sulfate catalyzed chromium plating bath, and the second or outermost layer (Layer B) is electrodeposited from an aqueous bath formed in accordance with United States Patent No. 2,640,022 Stareck. The following Baths I and II will serve as specific examples, respectively, of suitable baths for electrodepositing Layers A and B:

Bath I z./ gal. CI'Os H 80 .40

Bath 11 CrO 26.0 K Cr O 4.9 SrCrO; 0.6 K SiF 1.4 SrSO 0.8

The initial chromium layer (Layer A) is deposited from Bath I under a cathode current density of approximately 140 amperes per square foot at a bath temperature of approximately 120 F. for approximately 5 minutes to 6 minutes to produce a minimum coating thickness of at least 0.000015 inch. The part is then thoroughly rinsed and directly passed into Bath II which is maintained at a temperature of approximately 120 F. The bath is electrolyzed at a cathode current density of approximately 125 amperes per square foot for a suflicient duration to obtain a minimum coating thickness of about 0.000015 inch.

Although optimum results are obtained by applying Layer A in the manner indicated above, improvements can be obtained using any of the bath formulations and conditions which are generally satisfactory for applying single coatings of chromium from a sulfate catalyzed bath. Generally Layer A can be formed in the normal and accepted manner of electrodepositing chromium from a sulfate catalyzed bath. In most instances highly satisfactory results are obtained with my invention if Layer A is deposited from the sulfate catalyzed bath in the manner which would provide best durability if Layer A were used as a single layer. The chromium coating of this example of my invention generally provides optimum results if Layer A is deposited with as sparse a crack pattern as possible.

Similarly, the composition of Bath H, as well as the conditions of using it, can be varied. The composition of Bath II and its conditions of use can vary generally as described in United States Patent No. 2,640,022 Stareck. These variations are as material to the subject invention as they are to conventional chromium plating to obtain maximum durability. In most instances optimum results are obtained when Layer B is deposited so as to have a relatively dense crack pattern when compared to Layer A.

It has also been found that the advantages of my invention can also be obtained using a bath composition made up in accordance with the aforementioned United States Patent No. 2,640,033 Stareck, hereinafter listed as Bath IV, in conjunction with a bath composition and method of chromium plating such as described in United States Patent No. 2,916,424 Stareck et al., hereinafter listed as Bath III. The immediately following disclosure of Baths II and IV and the method of using same is presented to serve as a specific example of this embodiment of the invention.

The initial chromium layer (Layer A) is deposited from Bath IH under a cathode current density of approximately 220 amperes per square foot at a bath temperature of approximately F. for a sufiicient duration to produce a minimum coating thickness of about 0.000015 inch. The bath was then thoroughly rinsed and directly passed into Bath IV which was maintained at a temperature of approximately 120 F. Bath IV was then electrolyzed at a cathode current density of approximately 200 amperes per square foot for a sufiicient duration to obtain a minimum coating thickness of approximately 0.000015 inch.

The sulfate ion concentration in Bath III was attained by adding suitable amounts of sulfuric acid to the bath solution which was previously saturated with strontium sulfate. The silicofluoride ion concentration was attained using K SiF The aforementioned United States Patent No. 2,916,424 Stareck et al. is specifically directed to providing a process for plating crack-free chromium surfaces of mirror-brightness to a thickness of at least 0.00005 inch. However, it has been found that unexpectedly improved durability of chromium plated parts can be attained with the process described in the abovementioned Stareck et al. patent, when used in my invention, even if it is modified by depositing a lesser thickness of chromium. As indicated above, significantly improved results have been obtained when the deposit of the Layer A chromium is only about 0.000015 inch in thickness.

The aforementioned United States Patent No. 2,916,424 Stareck et a1. discloses a relationship pertinent to Bath III between temperature, concentration and ratio, such as illustrated by FIGURES 1 and 2. The sulfate and silicofluoride ions in Bath HI are referred to as catalyst ions. The term ratio as used in the discussion of FIGURES 1 and 2 refers to the ratio of the weight of CrO to the weight of the catalyst ions in the bath.

Stareck et al. disclose that bright, crack-free chromium can be electroplated at temperatures between 112 F. and 119 F. as shown in FIGURE 1 by using a high ratio. FIGURE 1 also indicates that at 115 F. a ratio of about 115:1 is the limiting ratio whereas at 112 F. it is 150:1. It is further stated that in many combinations of the three variables; temperature, concentration and ratio, electrodeposition of bright, crack-free chromium between the temperatures of 117 F. and 135 F. is preferred. It is preferred to utilize CrO concentrations between about 250 grams per liter and about 425 grams per liter within this latter temperature range; the relationship between the three variables being defined by the area enclosed by the curve in FIGURE 2. Within the enclosed area ABC in FIGURE 2, ratiosof 80:1 and 150:1 may be used. Within the area defined as ABD, the preferred range of ratios is narrower with the limiting preferred range at the line ADB being 115:1 to 150:1.

It is further stated that bright, crack-free chromium can be deposited at a maximum thickness when plating with the rectangular area ABCD of FIGURE 1 at ratios between about 105:1 to 135:1, wherein baths having a CrO concentration between about 275 grams per liter and 400 grams per liter and plating temperatures of about 125 F. and 138 F. are used.

When the temperatures and the CrO concentrations are maintained within the area defined by the closed curve ADFG in FIGURE 1, it is stated as possible to deposit bright, crack-free chromium in all ratios between 80:1 and 150: 1.

The advantages of the invention have also been obtained by applying a chromium coating having a plurality of alternately different chromium layers using two sulfate catalyzed chromium plating baths having compositions as follows:

The initial chromium layer (Layer A) was deposited from Bath V, which was at a temperature of approximately 102 F. to 106 F., under a cathode current density of approximately 108 amperes per square foot. The bath was electrolyzed for approximately 7 /2 minutes to apply a minimum coating thickness of about 0.000015 inch. The part was then removed from Bath V, thoroughly rinsed and placed in Bath VI, which was at a temperature of approximately 122 F. to 126 F. The second layer (Layer B) was deposited from Bath VI under a cathode current density of 144 amperes per square foot for approximately 5 minutes to obtain the minimum Layer B thickness of about 0.000015 inch.

Layer A should be deposited with a relatively parse crack pattern while Layer B should be deposited with a comparatively dense crack pattern. Baths V and VI can be varied to some extent without losing the benefits of the invention. For example Layer A and Layer B can, respectively, be satisfactorily formed using bath compositions and conditions such as follows:

Bath V (A) Chromic acid oz./gal 20-35 Sulfuric acid oz./gal .20-.35 Temperature PL. 95-115 Average cathode current density ASF 90-140 6 Bath VI (A) Chromic acid oz./gal 40-70 Sulfuric acid 0z./gal... .45.70 Temperature F -150 Average cathode current density ASF 130-300 As previously indicated, the material advantages of the invention are obtained with a coating composed of alternate layers of two dilferent types of chromium deposits. These advantages can be obtained with two or more layers of chromium provided that the necessary relationships between contacting chromium layer are present. In analogous arrangement, in certain instances, more than two distinctly different types of deposits may be used to obtain the unexpected benefits. The different types of deposits may be formed with different or similar bath solutions. It is known that a plurality of chromium coatings having significantly different characteristics may be deposited from the same bath by adjusting the conditions of deposition. Should these latter coatings be of the required character, they may be used to practice the invention.

The peculiar durability of the product of my invention appears to be associated with relative crack pattern and nobility characteristics of the chromium layers. Moreover, the relative nobility of the chromium layers to a subjacent nickel layer is believed to be important to obtain optimum durability of steel parts without corrosion failure to red rust. By the use of the term noble or nobility I refer to relative differences between the electromotive potentials of two metals. A metal of lesser nobility than another would be one which would exhibit an anodic potential with the other metal when the two metals comprise a voltaic cell.

Especially high corrosion resistance was obtained when one of the chromium layers, particularly Layer A, was of a lesser nobility than the activated nickel layer. Satisfactory results may be obtained when both Layers A and B are of a lesser nobility than the activated nickel layer. However, optimum results were obtained when Layer B was more noble than the nickel layer and Layer A was less noble than the nickel layer.

It is generally preferred that there be differences in the nobility of the respective chromium layers as they exist in the multilayer coating. In most instances best results were obtained when these differences were approximately 0.025 volt to 0.150 volt and in certain instances difference as great as 0.175 volt may be used. Moreover, it is preferred that Layer A, when active, be less noble than the nickel layer, when active. Differences in electromotive potential between these latter two layers, when activated, from about 0.025 volt to 0.10 volt and even as high as 0.13 volt are useful in obtaining the advantages of the invention.

Moreover, it is generally preferred that the two chromium layers of a dual chromium coating have different crack patterns. Preferably one of the layers should have a dense microscopic crack pattern while the other of the chromium layers has a comparatively sparse crack pattern. Improved results can be obtained if Layer A has a fine crack pattern and Layer B is virtually crack-free. However, optimum results are obtained when the outermost chromium layer (Layer B) has the fine crack pattern and Layer A has a comparatively sparse crack pattern. Although it is preferred that Layer B have a crack pattern which is approximately 30 to100 times denser than that of Layer A, in some instances it may be preferred to employ for Layer B a crack pattern which is only 5 times denser than that of Layer A or as much as times denser than that of Layer A. Improved results may be obtained when Layer B has from about 50 to 1200 cracks p er lineal inch.

Most generally I prefer to employ the more favorable relative crack pattern relationship and nobility relationship hereinafter described in combination to obtain opti- 8 outermost layer being about 50 cracks to 1200 cracks per lineal inch. I

. 4. An article having a decorative corrosion-resistant coating thereon which includes a nickel coating and a dual layer chromium coating on the nickel coating, said chromium layers having a thickness of at least about 0.000015 inch but less than 0.000050 inch and the outermost of said layers exhibiting a crack pattern at least 30 times denser than the crack pattern of the innermost layer, the crack pattern density of said outermost layer being at least about 50 cracks per lineal inch.

, 5. An article having a decorative corrosion-resistant coating thereon which includes a nickel coating and a dual layer chromium coating on the nickel coating, said chromium layers having a thickness of at least about 0.000015 inch but less than 0.000050 inch and the outermost of said layers exhibiting a crack pattern at least five times denser than the crack pattern of the innermost layer, the crack pattern density of said outermost layer being about 50 cracks to 1200 cracks per lineal inch.

mum durability of the chromium plated workpiece and optimum consistency in obtaining improved durability.

Although this invention has been described in connection with certain specific examples thereof, no limitation is intended thereby except as defined in the appended claims.

I claim:

1. An article having a decorative corrosion-resistant metal coating thereon which includes a nickel coating and a dual layer chromium coating on the nickel coating, the outermost of said chromium layers having a thickness of at least about 0.000015 inch but less than 0.000050 inch and exhibiting a crack pattern denser than the crack pattern of the innermost layer, the crack pattern density of said outermost layer being at least about 50 cracks per lineal inch.

2. An article having a decorative corrosion-resistant coating thereon which includes a nickel coating and a dual layer chromium coating on the nickel coating, the outer most of said chromium layers having a thickness of at 20 least about 0.000015 inch but less than 0.000050 inch and exhibiting a crack pattern at least about five times denser than the crack pattern of the innermost layer, the crack pattern density of said outermost layer being at least about References Cited by the Examiner UNITED STATES PATENTS 50 cracks per lineal inch. 2,678,908v 54 Tucker 20441 3. An article having a decorative corrcsion-resistant 2,678,909 5/54 tedt 204-41 coating thereon which includes a nickel coatting and a 2,800,443 7/57 r k 29--196.6 dual layer chromium coating on the nickel oatin id 2,916,424 12/ 59 Stareck 20451 layers havingathickness of at least about 0,000015 in 9 9/60 r k 204-51 but less than 0.000050 inch and the outermost layer ex- DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, Examiner.

UNITED STATESPATENT OFFICE QERTIFICATE 0F CORRECTION Patent No. 3,188,186

June 8, 1965 Rollie P. Durham It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as correctedbelow.

Column 2, line 42, for "an" read a lin e 67 for "seonds" read seconds column 4, line 24, for "2 640 033" read 2,640,022 line 29, for "II" read Ill column 7, llne 27, for "coatting" read coating Signed and sealed this 7th day of December 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENN Attcsting Officer ER Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 188 186 June 8 1965 Rollie P. Durham It is hereby certified that error appears in the above numbered patent reqiiring correction and that the said Letters Patent should read as correctedbelow.

Column 2, line 42, for "an" read a line 67, for "seonds" read seconds column 4, line 24, for "2,640,033 read 2,640,022 line 29, for "II" read III column 7, line 27, for "coatting" read coating Signed and sealed this 7th day of December 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Aitcsting Officer Commissioner of Patents 

1. AN ARTICLE HAVING A DECORATIVE CORROSION-RESISTANT METAL COATING THEREON WHICH INCLUDES A NICKEL COATING AND A DUAL LAYER CHROMIUM COATING ON THE NICKEL COATING, THE OUTERMOST OF SAID CHROMIUM LAYERS HAVING A THICKNESS OF AT LEAST ABOUT 0.000015 INCH BUT LESS THAN 0.000050 INCH AND EXHIBITING A CRACK PATTERN DENSER THAN THE CRACK PATTERN OF THE INNERMOST LAYER, THE CRACK PATTERN DENSITY OF SAID OUTERMOST LAYER BEING AT LEAST ABOUT 50 CRACKS PER LINEAL INCH. 